Intersystem change between different radio access networks

ABSTRACT

A wireless terminal can communicate with two radio access networks (RAN) ( 304, 306 ) of different types. The wireless terminal can register simultaneously with a RAN of the first type and a RAN of the second type and can wirelessly connect to a RAN of either the first type or the second type in a connected state. The wireless terminal when registered with both a first RAN and a second RAN and when wirelessly connected to the first RAN in the connected state, receives a mobility signal via the first RAN indicating a RAN of the second type and, in response to receiving the mobility signal and in response to being so registered with the first RAN and second RAN, transmits an access trigger signal ( 417 ). The access trigger signal indicates that the wireless terminal is to be connected to the second RAN in the connected state.

TECHNICAL FIELD

The invention relates to intersystem change and in particular to animproved intersystem change of a wireless terminal between differentradio access networks.

BACKGROUND ART

Radio access networks (RANs) of different wireless cellularcommunication systems operate according to various different definedsets of standards each having different respective radio accesstechnologies (RAT). Wireless terminals operated by users of such systemsare usually configured so that they can operate with more than one suchradio access network. Therefore it is often required that a terminalmust be able to connect to more than one type of network operatingaccording to different respective standards, and must be able to switchdynamically between such different networks. Such a switching process isoften termed an inter-RAT ‘handover’.

GSM (Global System for Mobile Communications, originally Groupe SpecialMobile), is a set of standards developed by the EuropeanTelecommunications Standards Institute (ETSI) to define technologies forso-called second generation (2G) digital cellular networks. Developed asa replacement for first generation (1G) analog cellular networks, theGSM standard originally described a digital, circuit switched networkoptimized for full duplex voice telephony. The standard was expandedover time to include first circuit switched data transport, then packetdata transport via GPRS (General Packet Radio Services).

Enhanced Data rates for GSM Evolution (EDGE) (also known as EnhancedGPRS (EGPRS)) is a digital mobile telephone technology that is abackward-compatible extension of GSM and allows improved datatransmission rates. EDGE is considered a pre-third generation (3G) radiotechnology and was deployed on GSM networks beginning in 2003. EDGE isstandardized by the international organisation known as 3GPP (3rdGeneration Partnership Project) as part of the GSM family. A networkthat operates according to the GSM and/or EDGE standards is known as aGSM/EDGE radio access network (GERAN).

Universal Mobile Telecommunications System (UMTS) is a third generationmobile cellular technology for communication networks based on the GSMstandard. Developed by 3GPP, UMTS is a component of a set of standardsspecified and maintained by the International Telecommunications Union(ITU), this set of standards known as IMT-2000. IMT-2000 is comparableto, but different from, the CDMA2000 set of standards for networks basedon the competing cdmaOne™ technology deployed in the USA andinternationally elsewhere.

UMTS employs Wideband Code Division Multiple Access (W-CDMA) radioaccess technology to offer greater spectral efficiency and bandwidth.UMTS specifies a complete network system, covering the radio accessnetwork (UMTS Terrestrial Radio Access Network, or UTRAN), the corenetwork (Mobile Application Part, or MAP) and the authentication ofusers via SIM cards (Subscriber Identity Module).

UMTS and GSM/EDGE can share a Core Network (CN), making UTRAN analternative radio access network to GERAN, and allowing (mostly)transparent switching between these radio access networks (RANs)according to available coverage and service needs. Because of that, UMTSand GSM/EDGE radio access networks are sometimes collectively referredto as UTRAN/GERAN. Most cells of European mobile cellular communicationsystems, and most handsets or mobile terminals used in such networks,can support both UTRAN and GERAN operation.

Since 2006, UMTS networks in many countries have been or are in theprocess of being upgraded with High Speed Downlink Packet Access(HSDPA), sometimes known as 3.5G. Currently, HSDPA enables downlinktransfer speeds of up to 21 Mbit/s. Work is also progressing onimproving the uplink transfer speed with the High-Speed Uplink PacketAccess (HSUPA). Longer term, the 3GPP Long Term Evolution (LTE™) set ofstandards, termed fourth generation (4G), aims to provide data transferrates of 100 Mbit/s on the downlink and 50 Mbit/s on the uplink, using a4G air interface technology based upon orthogonal frequency-divisionmultiplexing (OFDM).

E-UTRAN is an abbreviation for evolved UMTS Terrestrial Radio AccessNetwork.

E-UTRAN or eUTRAN is the radio access network defined by the LTE™standards.

E-UTRAN uses a simplified single node architecture consisting of eNBs(E-UTRAN Node B)—see FIG. 1.

Referring to FIGS. 1 and 2, an eNB 102 communicates with an EvolvedPacket Core (EPC) 202 using an S1 interface 104. Specifically the eNB102 communicates with a MME (Mobility Management Entity) node 106, 206and a UPE (User Plane Entity) node identified here as a S-GW (ServingGateway) 108, 208 using S1-C and S1-U interfaces 104 for control planeand user plane respectively. The MME node 106, 206 and the UPE node 108,208 are preferably implemented as separate network nodes so as to allowindependent scaling of the control and user plane. Also any eNB cancommunicate with other eNBs using an X2 interface (X2-C and X2-U forcontrol and user plane respectively). eNBs transmit signals to, andreceive signals from, wireless terminals or ‘user equipments (UEs). Thusa wireless terminal can be connected to the MME via a eNB.

A HSS (Home Subscriber Server) (not shown) is a central database thatcontains user-related and subscription-related information. Functions ofthe HSS include mobility management, call and session establishmentsupport, user authentication and access authorization.

The MME (Mobility Management Entity) is the key control-node for theE-UTRAN access network. It is responsible for idle mode UE (UserEquipment) tracking and paging procedures including retransmissions. Itis involved in bearer activation/deactivation processes and is alsoresponsible for choosing the serving gateway (S-GW) for a UE when the UEinitially attaches to the network and during intra-E-UTRAN handoverinvolving Core Network (CN) node relocation. It is responsible forauthenticating the user (by interacting with the HSS).

Non Access Stratum (NAS) signalling terminates at the MME and the MME isresponsible for generation and allocation of temporary identities toUEs. The MME checks the authorization of the UE to register with theservice provider's Public Land Mobile Network (PLMN) and enforces UEroaming restrictions. The MME also provides the control plane functionfor mobility between LTE and UTRAN/GERAN access networks with the S3interface terminating at the MME from the SGSN of the UTRAN/GERANnetwork.

The Serving GPRS Support Node (SGSN) of a UTRAN/GERAN network has asimilar overall function to that of the MME of the E-UTRAN network. TheSGSN is responsible for the delivery of data packets from and to thewireless terminals (‘mobile stations’) within its geographical servicearea. Its tasks include packet routing and transfer, mobility management(attach/detach and location management), logical link management, andauthentication and charging functions. The location register of the SGSNstores location information (e.g., current cell, current visitorlocation register, VLR) and user profiles (e.g., IMSI, address(es) usedin the packet data network) of all GPRS users registered with the SGSN.

Among other functions, the SGSN performs functions associated withmobility management required when a wireless terminal in standby modemoves from one Routing Area (RA) to another Routing Area.

E-UTRAN specifies an Idle mode Signaling Reduction (ISR) function whichprovides a mechanism to limit or reduce signaling in idle mode duringany inter-RAT cell-reselection between E-UTRAN and UTRAN/GERAN.According to this mechanism a wireless terminal (User Equipment, UE) inidle mode, when ISR is activated, is registered with both the MME of theE-UTRAN and the SGSN of a UTRAN/GERAN (see 3GPP TS23.401, Annex J1).Both the SGSN and the MME have a control connection with the servinggateway (S-GW). The MME and SGSN are both registered at the HSS. The UEreceives and stores mobility management (MM) parameters provided to theUE by the SGSN (e.g. P-TMSI and RA) and provided to the UE by the MME(e.g. GUTI and TA(s)) and the UE stores session management (bearer)contexts that are common to E-UTRAN and GERAN/UTRAN accesses.

Using these stored parameters and contexts, the UE when it is in idlestate can reselect between E-UTRAN and GERAN/UTRAN radio access cellswhen the UE is within the registered radio access routing areas (RAs)and tracking areas (TAs) without any need to perform any tracking areaupdate (TAU) or radio access update (RAU) procedures by interacting withthe network.

CITATION LIST Non Patent Literature

-   NPL 1: 3GPP TS 23.401 V9.0.0 (2009-03)

SUMMARY OF INVENTION Technical Problem

Situations sometimes occur which cause unsynchronized state informationin one or more of the UE, MME and SGSN (see 3GPP TS23.401, annex J6).There are no ISR-specific procedures that could handle such specialsituations and could thereby avoid additional complexity and errorcases. All such special situations that would cause context in the UE,MME and SGSN to become asynchronous are handled by de-activation of theISR function. The de-activation of the ISR function involvesde-registering the UE from the RAN other than the RAN to which the UE isconnected in connected mode. The known RAU and/or TAU procedures thenserve to synchronize contexts between the MME and the SGSN and, when thesynchronization is done, the ISR function is activated again whendesired by the network.

Another situation which causes a problem during an inter-RAT handoverhas not been appreciated until now, and is illustrated in FIG. 3.

FIG. 3. is an event diagram showing operations (represented by boxes)performed by entities (wireless terminal 302 for example a mobileterminal, E-UTRAN MME 304 and GERAN SGSN 306) wherein time isrepresented by a downward vertical direction (not to scale). One or moresignals between entities are represented by horizontal arrows. In thisexample only one signal is shown and only operations performed by thewireless terminal are shown.

In box 310, the wireless terminal 302 is connected to the E-UTRAN MME bymeans of signalling that takes place between the wireless terminal andthe E-UTRAN MME. As a result of this connection, the wireless terminalis in a connected state in which the wireless terminal can transmit andreceive traffic data via the E-UTRAN. The wireless terminal isconsidered to be in E-UTRAN connected mode. ISR is active or activatedi.e. ON in the wireless terminal and therefore the wireless terminal isregistered with both the E-UTRAN and the GERAN, the registration of thewireless terminal being associated with a registration area (TrackingArea, TA) by the E-UTRAN and also being associated with a registrationarea (Routing Area, RA) by the GERAN. The GERAN associates the wirelessterminal with a registration area/routing area ‘RAF.

As indicated by arrow 312, the wireless terminal receives a signalcomprising an indication in the form of a message termed ‘Mobility fromE-UTRAN Command’ having a ‘purpose’ parameter set to ‘Cell Change Order’(CCO) and a target RAT type set to GERAN as specified in 3GPP TS36.331,s5.4.3.3. The message also indicates a target GERAN cell (this beinglocated within routing area RA1).

In box 314, the wireless terminal starts the indicated CCO procedure andalso starts a designated timer T3174 for CCO completion control and theterminal performs a reselection which serves to re-select the terminalto the target GERAN cell using the information provided in the ‘Mobilityfrom E-UTRAN Command’ message. In this situation, the target GERAN cellis within the same RA1 with which the wireless terminal is associated byits registration (that is, the target GERAN cell is within the samerouting area RA1 with which the wireless terminal is effectively alreadyregistered).

Because of this, and because ISR is ON, and if there is no pendinguplink data in the mobile terminal, there is no trigger for access tothe GERAN cell (for example, a Routing Area Update request message).Such a trigger is required for a successful completion of the CCOprocedure as specified in 3GPP TS44.060, s8.4.1. This represents aproblem.

In box 316, the timer T3174 expires and in box 318, the CCO procedurefails.

The present invention seeks to address the problem represented by theabove-described CCO-to-GERAN failure when the mobile terminal is inE-UTRAN connected mode, ISR is ON and the RA is not changed, and seeksto provide a more robust and reliable intra-cell handover betweendifferent radio access networks.

Solution to Problem

According to the invention there is provided a wireless terminal asclaimed in Claim 1.

The claimed subject matter provides a more robust and reliableinter-cell inter-RAT handover between different radio access networksand is well suited for use in the performing of an inter-RAT handoverbetween a E_UTRAN network and a UTRAN or GERAN network.

Embodiments will now be described, by way of example only, withreference to the accompanying drawings.

Advantageous Effects of Invention

The present invention can provide to address the problem represented bythe above-described CCO-to-GERAN failure when the mobile terminal is inE-UTRAN connected mode, ISR is ON and the RA is not changed, and providea more robust and reliable intra-cell handover between different radioaccess networks.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a simplified representation of a node architecture of anE-UTRAN network;

FIG. 2 is a simplified representation of various elements of an E-UTRANnetwork;

FIG. 3 is an event diagram showing operations performed by entities ofan E-UTRAN network according to current state of the art;

FIG. 4 is an event diagram showing operations performed by entities ofan E-UTRAN network according to an embodiment;

FIG. 5 is an event diagram showing operations performed by entities ofan E-UTRAN network according to another embodiment.

DESCRIPTION OF EMBODIMENTS

Turning to FIG. 4, an event diagram shows operations (represented byboxes) performed by entities (wireless terminal 302 for example a mobileterminal, E-UTRAN MME 304 and GERAN SGSN 306) wherein time isrepresented by a downward vertical direction (not to scale). One or moresignals between entities are represented by horizontal arrows. In thisexample only one signal is shown and only operations performed by thewireless terminal are shown.

In box 310, the wireless terminal 302 is connected to the E-UTRAN MME bymeans of signalling that takes place between the wireless terminal andthe E-UTRAN MME. As a result of this connection, the wireless terminalis in a connected state in which the wireless terminal can transmit andreceive traffic data via the E-UTRAN to which it is connected. Thewireless terminal considered to be in connected mode. ISR is active oractivated i.e. ON in the wireless terminal and therefore the wirelessterminal is registered with both the E-UTRAN and the GERAN, theregistration of the wireless terminal being associated with aregistration area by the E-UTRAN and also being associated with aregistration area by the GERAN. The GERAN associates the wirelessterminal with a registration area ‘RAF.

As indicated by arrow 312, the wireless terminal receives a signalcomprising an indication in the form of a message termed ‘Mobility fromE-UTRAN Command’ having a ‘purpose’ parameter set to ‘Cell Change Order’(CCO) and a target RAT type set to GERAN as specified in 3GPP TS36.331,s5.4.3.3. The message also indicates a target GERAN cell or RA (thisbeing located within RA1).

In box 314, the wireless terminal starts the indicated CCO procedure andalso starts a designated timer T3174 for CCO completion control and theterminal performs a reselection which serves to re-select the terminalto the target GERAN cell using the information provided in the ‘Mobilityfrom E-UTRAN Command’ message. In this situation, the target GERAN cellis within the same RA1 with which the wireless terminal is associated byits registration (that is, the target GERAN cell is within the samerouting area RA1 with which the wireless terminal is effectivelyregistered).

As indicated by arrow 417, the wireless terminal then triggers a routingarea update (RAU) as soon as it ‘camps’ successfully on the target GERANcell.

The wireless terminal can trigger RA update regardless of any pendinguplink data or request. Alternatively the wireless terminal triggers RAupdate provided that the registered RA is not changed (during theintersystem change from E-UTRAN to GERAN) and there is no pending uplinkdata or request.

In box 419, the CCO is successfully completed as the RAU by the mobileterminal completes the CCO procedure according to the requirementsspecified in TS44.060, s 8.4.1 which says:

“The mobile station shall regard the network controlled cell reselectionprocedure as successfully completed when it has performed access andsuccessfully completed contention resolution in the new cell.”

In box 420, the wireless terminal then stops the timer T3174.

Turning to FIG. 5, an event diagram, similar to that shown in FIG. 4,shows operations (represented by boxes) performed by entities (wirelessterminal 302 for example a mobile terminal, E-UTRAN MME 304 and GERANSGSN 306) wherein time is represented by a downward vertical direction(not to scale). One or more signals between entities are represented byhorizontal arrows. In this example only one signal is shown and onlyoperations performed by the wireless terminal are shown.

In box 310, the wireless terminal 302 is connected to the E-UTRAN MME bymeans of signalling that takes place between the wireless terminal andthe E-UTRAN MME. As a result of this connection, the wireless terminalis in a connected state in which the wireless terminal can transmit andreceive traffic data via the E-UTRAN node to which it is connected. Thewireless terminal is considered to be in E-UTRAN connected mode. ISR isactive or activated i.e. ON in the wireless terminal and therefore thewireless terminal is registered with both the E-UTRAN and the GERAN, theregistration of the wireless terminal being associated with aregistration area or cell by the E-UTRAN and also being associated witha registration area or cell by the GERAN. The GERAN associates thewireless terminal with a registration area ‘RA1’.

As indicated by arrow 312, the wireless terminal receives a signalcomprising an indication in the form of a message termed ‘Mobility fromE-UTRAN Command’ having a ‘purpose’ parameter set to ‘Cell Change Order’(CCO) and a target RAT type set to GERAN as specified in 3GPP TS36.331,s5.4.3.3. The message also indicates a target GERAN cell or RA (thisbeing located within RA1).

In box 314, the wireless terminal starts the indicated CCO procedure andalso starts a designated timer T3174 for CCO completion control and theterminal performs a reselection which serves to re-select the terminalto the target GERAN cell using the information provided in the ‘Mobilityfrom E-UTRAN Command’ message. In this situation, the target GERAN cellis within the same RA1 with which the wireless terminal is associated byits registration (that is, the target GERAN cell is within the samerouting area RA1 with which the wireless terminal is effectivelyregistered).

In box 515, the wireless terminal locally deactivates ISR, in thisexample by setting its TIN to “GUTI”, as soon as it successfullyreselects to (i.e. camps on) the target GERAN cell.

The wireless terminal can deactivate ISR regardless of any pendinguplink data or request. Alternatively the wireless terminal shalldeactivate ISR provided that the registered RA is not changed (duringthe intersystem change from E-UTRAN to GERAN) and there is no pendinguplink data or request.

De-activating the ISR function, when the RA is not changed and there isno pending uplink data or request, serves to overcome a problem that iscaused by the situation in which the RA is not changed and there is nopending uplink data or request. As already outlined above it has nowbeen recognized that, in this situation, there exists no trigger thatwould cause an initiation of an access to the target GERAN cell (RoutingArea update request message for example) needed for the successfulcompletion of the CCO (intersystem change).

As indicated by arrow 417, the wireless terminal then triggers a routingarea update (RAU) procedure as a result of the intersystem (inter-RAT)change and as a result of the local ISR deactivation.

The wireless terminal can trigger RA update regardless of any pendinguplink data or request. Alternatively the wireless terminal triggers RAupdate provided that the registered RA is not changed and there is nopending uplink data or request.

The wireless terminal triggering a routing area update, when the RA isnot changed and there is no pending uplink data or request, serves toovercome a problem that is caused by the situation in which the RA isnot changed and there is no pending uplink data or request. As alreadyoutlined above, it has now been recognized that in this situation thereexists no trigger that would cause an initiation of an access to thetarget GERAN cell (Routing Area update request message for example)needed for the successful completion of the CCO (intersystem change).

In box 419, the CCO is successfully completed as the RAU by the mobileterminal completes the CCO procedure according to the requirementsspecified in TS44.060, s 8.4.1 which says:

“The mobile station shall regard the network controlled cell reselectionprocedure as successfully completed when it has performed access andsuccessfully completed contention resolution in the new cell.”

In box 420, the wireless terminal then stops the CCO completion controltimer T3174.

If the CCO completion control timer T3174 expires after the mobileterminal has locally deactivated ISR but before CCO completion (accessto GERAN with successful completed contention resolution in the newcell) then the wireless terminal cancels the local ISR deactivation bysetting its TIN back to ‘RAT related TMSI’ before it goes back to thesource E-UTRAN cell and initiates connection re-establishment asspecified in TS36.331, s5.4.3.5.

The inventive features described above and illustrated by theaccompanying drawings allow successful CCO completion in a situation inwhich a wireless terminal performs an intersystem change from aE-UTRAN-connected state to a GERAN cell by CCO, when ISR is active,within the same RA and there is no pending uplink data or request. Thisallows the wireless terminal to consistently and reliably succeed inperforming the intersystem change in the above-described situation whichwould otherwise result in a failure and a negative user experience.

The inventive features serve to align a E-UTRAN to GERAN intersystemchange via CCO in connected mode to a E-UTRAN to GERAN intersystemchange via handover in connected mode since, according to the claimedinventive features, in each of these intersystem changes the wirelessterminal will trigger a RA update after successful selection to GERAN,regardless of the ISR status. The level of alignment depends on whetherthe pending uplink data or request in CCO is considered or not.

It should be understood that the invention may be implemented insoftware and/or in hardware. If it is implemented in software, thesoftware may be provided on a carrier medium such as, for example, CDROM, electronic memory or a signal transmitted over a computer network.The software may be integrated with software or programming code forperforming functions other that the functions relating directly to theclaimed features. The software may not exist as a separate module and itmay be integrated with an operating system for a computer or processor.

This application is based upon and claims the benefit of priority fromUnited Kingdom patent application No. 1205827.7, filed on Mar. 30, 2012,the disclosure of which is incorporated in its entirety by reference.

INDUSTRIAL APPLICABILITY

The invention relates to intersystem change and in particular to animproved intersystem change of a wireless terminal between differentradio access networks.

REFERENCE SIGNS LIST

-   102 eNB-   106 MME/S-GW-   108 MME/S-GW-   202 EPC-   206 MME-   208 S-GW-   302 wireless terminal-   304 E-UTRAN MME-   306 GERAN SGSN

1. A method comprising: receiving a mobility command from a first Radio Access Network (RAN), performing a cell change order to a second RAN; and deactivating, when a user equipment (UE) moves from the first RAN connected mode to the second RAN via the cell change order, an Idle mode Signaling Reduction (ISR) by setting a temporary identifier (TIN) to globally unique temporary identifier (GUTI) before initiating a routing area (RA) update procedure.
 2. The method according to claim 1, wherein the UE is configured to be registered with both a first apparatus via the first RAN and a second apparatus via the second RAN.
 3. The method according to claim 1, wherein the first RAN is Evolved Universal Terrestrial Radio Access Network (E-UTRAN) and the second RAN is GSM EDGE Radio Access Network (GERAN).
 4. The method according to claim 1, wherein the RA update procedure is initiated by an access trigger signal, the access trigger signal being a RAU (Routing Area Update) message.
 5. The method according to claim 2, wherein the first apparatus is Mobility Management Entity (MME), and wherein the second apparatus is Serving GPRS Support Node (SGSN).
 6. A user equipment (UE) comprising: a receiver configured to receive a mobility command from a first Radio Access Network (RAN), a processor configured to perform a cell change order to a second RAN, wherein the processor configured to deactivate, when the UE moves from the first RAN connected mode to the second RAN via the cell change order, an Idle mode Signaling Reduction (ISR) by setting a temporary identifier (TIN) to globally unique temporary identifier (GUTI) before initiating a routing area (RA) update procedure.
 7. The user equipment according to claim 6, wherein the UE is configured to be registered with both a first apparatus via the first RAN and a second apparatus via the second RAN.
 8. The user equipment according to claim 6, wherein the first RAN is Evolved Universal Terrestrial Radio Access Network (E-UTRAN) and the second RAN is GSM EDGE Radio Access Network (GERAN).
 9. The user equipment according to claim 6, wherein the RA update procedure is initiated by an access trigger signal, the access trigger signal being a RAU (Routing Area Update) message.
 10. The user equipment according to claim 7, wherein the first apparatus is Mobility Management Entity (MME), and wherein the second apparatus is Serving GPRS Support Node (SGSN). 